1. Field of the Invention
The present invention relates to a UV-curable ink preferably used in ink-jet recording, an ink-jet recording method improving image quality, and a novel photopolymerization initiator having high solubility in water.
2. Description of the Related Art
Examples of aqueous ink-jet inks in use include an aqueous black dye ink containing a water-soluble black dye dissolved in an aqueous medium, and an aqueous black pigment ink containing a black pigment such, as carbon black, in an aqueous medium.
Recording using an aqueous black dye ink facilitates a shortened initial fixing time, yields a satisfactory image density, and causes little or no bleeding between this ink and color inks. The recorded image, however, does not generally have sufficient water resistance, smear resistance, and light durability. When the recorded image is covered with a marker pen (high-lighter), the recorded characters may blur. In particular, in documents including black characters and color images, mutual bleeding may occur between a black character and the adjacent color images and results in a lack of crispness of the characters. Such a problem requires appropriate solutions, for example, a reduced printing rate.
An aqueous black pigment ink tends to require a prolonged initial time for fixing the pigment to a recording medium, which inhibits improvement in printing rate. Furthermore, increased printing density causes decreased smear or scuff resistance; hence, ink transferred by scuff marks may cause contamination, or tracing of the printed section with a color marker pen may lessen the crispness of the printed characters. Aqueous black pigment inks in practical use readily cause bleeding with color inks; hence, the use of such inks may be restricted in documents containing characters and color images in some cases.
Various means have been disclosed for solving such technical problems. Examples of improvement in smear resistance of aqueous pigment inks include addition of water-soluble resins as binding resins in inks, addition of curable resins, and addition of microemulsion particles. A means for reducing the fixing time includes use of a solvent which accelerates the penetration of the ink into the recording medium, the solvent being capable of suppressing bleeding between black and color sites and suppressing the decrease in optical density of the characters. A method for avoiding color bleeding includes use of a black ink containing a cationic polymer and carbon black which suppresses mutual diffusion with an anionic color ink.
Although these methods can effectively improve individually targeted problems, they are insufficient overall improvement.
Furthermore, one of the typical means for improving the smear resistance of a printed image using aqueous inks includes use of a UV-curable ink. UV-curable inks have been used for marking in industrial apparatuses in known ink-jet recording technologies. These inks are nonaqueous inks using ketone solvents. UV-curable inks are also disclosed in Japanese Patent Publication No. 5-64667, Japanese Patent Application Laid-Open Nos. 5-186725, and 7-224241, and U.S. Pat. No. 4,978,969.
An ink disclosed in Japanese Patent Publication No. 5-64667 contains a pigment and a polymerizable polyacrylate having a functionality of three or more as essential components, and a ketone and an alcohol as primary solvents. An ink disclosed in Japanese Patent Application Laid-Open No. 5-186725 contains water and a photo-curable prepolymer and is used in ink-jet printers. An ink disclosed in Japanese Patent Application Laid-Open No. 7-224241 is an aqueous UV-curable pigment ink used in bubble jet processes. This ink contains a pigment, an aqueous polymeric compound for dispersing the pigment in an aqueous medium, a water-soluble monomer, an aqueous medium, and a polymerization initiator, and has a viscosity of 10 cps or less. U.S. Pat. No. 4,978,969 discloses a Fv method for forming an image by discharging an ink containing a UV-curable adhesive by a thermal ink-jet process.
The present inventors have further studied conventional photo-curable inks for ink-jet recording, taking into consideration that an ink-jet recording process using a photo-curable ink is a method that possibly can improve the above-mentioned problems totally. Conventional photo-curable inks generally contain polymerizable oligomers and photopolymerization initiators which are not at all soluble or are slightly soluble in aqueous media, and contain various water-soluble organic solvents to stabilize discharge in ink-jet recording. However, such organic solvents may affect the penetrability of the inks into recording media, and thus affect image quality. The present inventors have concluded that such organic solvents are one of the reasons why conventional photo-curable inks are not sufficient to improve the aforementioned problems totally.
Accordingly, it is an object of the present invention to provide a photo-curable ink which does not substantially affect ink-jet recording performance even when a polymerizable oligomer and a photo-initiator are contained in the ink as essential components.
It is another object of the present invention to provide an ink-jet recording method which facilitates formation of a high-quality image using a photo-curable ink.
It is still another object of the present invention to provide an ink-jet recording method which facilitates formation of a multicolor image using a photo-curable ink.
According to a first aspect of the present invention, there is provided an ink for ink-jet recording comprising a coloring agent, a polymerizable oligomer, a photopolymerization initiator and water, the photopolymerization initiator having a solubility in water of 3 percent by weight or more.
According to a further aspect of the present invention, there is provided an ink for ink-jet recording comprising a coloring agent, a polymerizable oligomer, a photopolymerization initiator and water, the polymerizable oligomer having at least two acryloyl groups and a solubility in water of 10 percent by weight or more.
According to a further aspect of the present invention, there is provided a method for forming an image on a recording medium comprising the steps of: (a) imparting an ink comprising a coloring agent, a polymerizable oligomer, a photopolymerization initiator having a solubility in water of 3 percent by weight or more, and water, on the surface of the recording medium; and (b) irradiating the ink on the recording medium with light.
According to a further aspect of the present invention, there is provided a method for forming an image on a recording medium comprising the steps of: (a) imparting an ink for ink-jet recording on the recording medium, the ink comprising a coloring agent, a polymerizable oligomer having at least two acryloyl groups and a solubility in water of 10 percent by weight or more, a photopolymerization initiator, and water, and (b) irradiating the ink on the recording medium with light.
In these aspects, the oligomer and the photopolymerization initiator in each photo-curable ink have high solubility in water; hence, addition of an organic solvent, which adversely affects the image quality, for dissolving these components in the ink is not substantially required. As a result, a high-quality image is obtainable.
According to a further aspect of the present invention, there is provided a photopolymerization initiator represented by the following general formula (1): 
wherein n is an integer of 1 or more, m is an integer of 0 or more, the sum of n and m is in a range of 1 to 8, and n is in a range of 1 to 8 when m is zero.
Since the photopolymerization initiator effectively forms radicals by ultraviolet light and has high solubility in water compared to conventional nonionic photopolymerization initiators, it can be preferably used for aqueous photo-curable ink-jet inks and aqueous photo-curable paints.
According to a still further aspect of the present invention, there is provided a method for recording a multicolor image including a black image by imparting a black ink and at least one color ink selected from a yellow ink, a magenta ink, and a cyan ink to a recording medium by an ink-jet process, comprising the steps of:
(a) imparting a black ink having any one of the following Compositions (i) to (iv) to the recording medium by an ink-jet process:
Composition (i)
at least one coloring agent selected from the group consisting of a combination of a black pigment and a water-soluble cationic polymeric compound for dispersing the black pigment in water, a cationic black dye, and a cationic self-dispersing carbon black, a nonionic or cationic polymerizable oligomer, a nonionic or cationic photopolymerization initiator having a solubility in water of 3 percent by weight or more, and water;
Composition (ii)
at least one coloring agent selected from the group consisting of a combination of a black pigment and a water-soluble cationic polymeric compound for dispersing the black pigment in water, a cationic black dye, and a cationic self-dispersing carbon black, a nonionic or cationic polymerizable oligomer having at least two acryloyl groups and having a solubility in water of 10 percent by weight or more, a nonionic or cationic photopolymerization initiator, and water;
Composition (iii)
at least one coloring agent selected from the group consisting of a combination of a black pigment and a water-soluble anionic polymeric compound for dispersing the black pigment in water, an anionic black dye, and an anionic self-dispersing carbon black, a nonionic polymerizable oligomer, a nonionic photopolymerization initiator having a solubility in water of 3 percent by weight or more, and water;
Composition (iv)
at least one coloring agent selected from the group consisting of a combination of a black pigment and a water-soluble anionic polymeric compound for dispersing the black pigment in water, an anionic black dye, and an anionic self-dispersing carbon black, a nonionic polymerizable oligomer having at least two acryloyl groups and having a solubility in water of 10 percent by weight or more, a nonionic photopolymerization initiator, and water;
(b) imparting a color ink having any one of the following Compositions (v) to (viii) to the recording medium by an ink-jet process:
Composition (v)
at least one coloring agent selected from the group consisting of a combination of a color pigment and a water-soluble cationic polymeric compound for dispersing the color pigment in water, and a cationic color dye, a nonionic or cationic polymerizable oligomer, a nonionic or cationic photopolymerization initiator having a solubility in water of 3 percent by weight or more, and water;
Composition (vi)
at least one coloring agent selected from the group consisting of a combination of a color pigment and a water-soluble cationic polymeric compound for dispersing the color pigment in water, and a cationic color dye, a nonionic or cationic polymerizable oligomer having at least two acryloyl groups and having a solubility in water of 10 percent by weight or more, a nonionic or cationic photopolymerization initiator, and water;
Composition (vii)
at least one coloring agent selected from the group consisting of a combination of a color pigment and a water-soluble anionic polymeric compound for dispersing the color pigment in water, and an anionic color dye, a nonionic polymerizable oligomer, a nonionic photopolymerization initiator having a solubility in water of 3 percent by weight or more, and water;
Composition (viii)
at least one coloring agent selected from the group consisting of a combination of a color pigment and a water-soluble anionic polymeric compound for dispersing the color pigment in water, and an anionic color dye, a nonionic polymerizable oligomer having at least two acryloyl groups and having a solubility in water of 10 percent by weight or more, a nonionic photopolymerization initiator, and water; and
(c) irradiating the black ink and the color ink imparted to the recording medium with light for photo-crosslinking of the inks, wherein the black ink and the color ink are selected so that the polarity of the coloring agent and the water soluble polymeric compound of the black ink are opposite to the polarity of the coloring agent and the water soluble polymeric compound of the color ink.
The method according to this aspect can effectively suppress bleeding between adjacent different color inks on the recording medium before photo-crosslinking, resulting in formation of a high-quality image.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.